1. Field of the Invention
The present invention relates to a variable wavelength laser light source which is capable of changing its wavelength without mode hopping, and to produce the light with a high resolution of the wavelength by eliminating the spontaneously emitted light.
This application is based on patent application No. Hei 09-329344 filed in Japan, the content of which is incorporated herein by reference.
2. Description of Related Art
A conventional example of the prior art, which is shown in FIG. 5, will be explained below. In FIG. 5, a semiconductor laser 1 with two faces 1a, 1b is provided. The face 1a is a reflecting face and the other face 1b has a non-reflection film on its surface. Diffraction grating 2 is located to one side of face 1b, and collimator lens 7 is between the face 1b and the diffraction grating 2. Total reflection mirror 3 is provided perpendicularly to the outgoing light of the desired wavelength which is diffracted by the diffraction grating 2, and reflects the diffracted light back to the diffraction grating 2. Supporting bar 5 connects the total reflection mirror 3 and rotating mechanism 4. Condenser lens 8 is provided between the semiconductor laser 1 and an optical fiber 6, and the optical fiber 6 is located over the face 1a of the semiconductor laser 1. Collimator lens 7 is also provided between the semiconductor laser 1 and the diffraction lens 2.
Point A is an intersection of the optical axis Ax and the diffraction face 2a. Virtual reflection face 9 is located at a distance from the point A toward the face 1a which depends on the refractive index n1 of semiconductor laser 1 and n2 of the collimator lens 7.
A line extended through the virtual reflection face 9 and a line extended through the diffraction face 2a intersect at the central axis C of the rotating mechanism 4.
Also, point B is the intersection of diffracted light emitted from the diffraction grating 2 and the total reflection mirror 3. .alpha. is the incident angle of the laser light emitted by the semiconductor laser 1, at the diffraction face 2a. .beta. is the output angle of the laser light emitted from the diffraction face 2a to the reflection face 3a of the total reflection mirror 3.
The physical length L from the point A to the face 1a is calculated by the formula (1); L1 is the distance between the point A and the face 1b, L2 is the length of a resonator in the semiconductor laser 1. EQU L=L1+L2 (1)
Also, the distance L' from the point A to the virtual reflection face 9 is calculated by the formula (2); L3 is a thickness of the collimator lens 7. EQU L'=(L1-L3)+L2.times.n1+L3.times.n2 (2)
In the formula (2), n1 is the refractive index of the semiconductor laser 1, and n2 that of the collimator lens 7.
Laser oscillation is performed by an optical oscillator composed by the face 1a and the total reflection mirror 3, and the semiconductor laser 1 which amplifies the light. Wavelength of the oscillated laser light is calculated by the formula (3). EQU SIN .alpha.+SIN .beta.=m.times.N.times..lambda. (3)
In the formula (3), m is the diffraction order at the diffraction grating 2 and N is the number of grooves per unit of length of the diffraction grating 2.
The total reflection mirror 3 is connected to the rotating mechanism 4 by the supporting bar 5. By rotating the rotating mechanism 4 around the central axis, the angle of the diffracted light D changes; D is incident perpendicularly to the reflection face 3a from the diffraction grating 2. For that reason, wavelength .lambda. of the oscillated laser light changes.
As explained above, by rotating the rotating mechanism 4, the position of the reflection face 3a changes, and consequently the wavelength .lambda. of the oscillated laser light emitted by the light source with variable wavelength can be changed.
Further, the light source with variable wavelength explained above has no mode-hopping when changing the wavelength .lambda. of oscillation, because the oscillation is performed under the condition that the longitudinal mode of the laser light is always constant. The method used in the above light source is the well-known SIN bar method.
There is a problem in the conventional light source with variable wavelength that the rotating mechanism 4 becomes very complicated and expensive when a light source with high resolution is desired and a rotating mechanism with high speed and high resolution is required, because high rotating resolution of the rotating mechanism is required when high resolution of the wavelength is needed, in the conventional light source with variable wavelength.
Another problem in the above conventional light source with variable wavelength is that, as shown in the FIG. 6, the accuracy of the output wavelength is degraded because the laser light with a wavelength selected by the diffraction grating 2 and spontaneously emitted light 10b from the semiconductor laser 1 are simultaneously output.